1. Field of the Invention
The present invention relates to voltage regulators and in particular, to a low drop-out voltage regulator with low power dissipation.
2. Description of the Related Art
Currently, the increasing demand for higher performance power supply circuits has resulted in a continued development of voltage regulator devices. Many low voltage applications, such as for use in cell phones, pagers, laptops, camera recorders and other mobile battery operated devices, require the use of low drop-out (LDO) voltage regulators. These portable electronics applications typically require low voltage and small quiescent current flow to increase the battery efficiency and longevity.
The LDO voltage regulators generally can provide a well-specified and stable DC voltage whose input to output voltage difference is low. The LDO voltage regulators are usually configured for providing the power requirements to electrical circuits. The LDO voltage regulators typically have an error amplifier, a dynamic bias circuit and a pass device, e.g., a power transistor. These three components are coupled in series. The error amplifier is coupled to an input terminal of the LDO voltage regulators, and the pass device is coupled to an output terminal of the LDO voltage regulators. The dynamic bias circuit is configured to drive the pass device, which can then drive an external load.
In general, a feedback circuit is further provided to the LDO voltage regulators scaling the output voltage down and feeding back a scaled down voltage to the error amplifier. The negative feedback provided by the feedback circuit can improve the stability of the regulator system. The LDO voltage regulators can further incorporate a compensation circuit to form a control loop and provide Miller compensation in order to improve the stability of the LDO voltage regulators. A conventional technique for providing Miller compensation is to take advantage of the Miller Effect, by adding a Miller compensation circuit or a nested Miller compensation (NMC) circuit which includes a Miller compensation capacitor. The Miller compensation capacitor is inserted between the output voltage and the error amplifier. Such a configuration may result in a well-known phenomenon called pole splitting, which advantageously multiplies the effective capacitance of the physical capacitor used in the circuit. However, the Miller compensation capacitor may cause the two poles to meet together, and then generate two complex poles in a right-hand plane along a direction, especially when the LDO voltage regulator covers a larger range of a capacitive load with an equivalent serial resistance (ESR) and provides a large output current. The right-hand plane poles can cause voltage oscillation at the LDO voltage regulators, which will make the output voltage unstable.
It is thus desirous to have an apparatus and method that can provide a stable output voltage when the capacitance of a load varies in a larger range, and at the same time output a corresponding current with low power dissipation, high driving capacity, and good stability. It is to such an apparatus and method the invention is primarily directed to.